What's the effect of Contact lens Base Curve?

I know what the base curve is and that it should closely match the curve of
your cornea, but how does a doctor decide which to use? E.g. 8.6 vs 8.9mm
seem really close -- how critical is it?

And, how does an incorrect base curve affect comfort? I have one lens at
8.6 that usually seems to feel "tight" -- would an 8.9 (flatter curve) be
better?

Keith

 

Keith,

A correct fitting is not only depending on a backcurve of a contactlens.
Design of the backcurve(s) , diameter of the lens , among other decisions
are to be made by a professional.
You are wrong in assuming that a backcurve of a contactlens has to be as
close as possible.
Fittings issues can be discussed in general but a fitting done by internet
directed to one persons eye is not possible.

Jan (normally Dutch spoken)

 

The fit of a lens is dependent upon the relationship between the cornea
curve and diameter, eye lid tension and thickness and then the lens base
curve, diameter, edge shape and material structural rigidity. Not as
simple as you might think.

An 8.6 in one lens brand might be tight, another loose. Same for 8.9.
Sometimes it's critical and sometimes not. Sometimes a "tight" lens
feels better than a loose lens, while damaging the peripheral cornea and
limbus. And sometimes not.

--LB

--
Dr. Larry Bickford, O.D.
Family Practice Eye Health & Vision Care

The Eyecare Connection
http://www.eyecarecontacts.com
larrydoc at eye-care-contacts dot com (remove -)

 

Okay, so how does one decide when to try a different base curve? Can you
give an example of symptoms that would indicate incorrect base curve? I
can't beleive a doctor can actually measure all the paramters you mentioned
(e.g. eye lid tension and thickness) and "calculate" which base curve is
correct -- he/she has to go by symtoms -- correct?

Keith

 

Knowing the eye parameters, lens parameters and patient's needs and
desires allow us to choose "best choice" lenses for trial. Then we
observe the lens on the eye and, based on objective observation and
wearer subjective feedback, decisions can be made on how to fine-tune
the lens fit.

If it feels good and you see well, it's likely a good lens fit. If it
hurts or you don't see well, then......well you can figure that out!
Having a competent eyecare professional who knows how to look for things
that might result in problems later on is the best approach. I think
that's pretty CLEAR, yes?

--LB

--
Dr. Larry Bickford, O.D.
Family Practice Eye Health & Vision Care

The Eyecare Connection
http://www.eyecarecontacts.com
larrydoc at eye-care-contacts dot com (remove -)
[website and email temporarily unavailable, probably until 7/5]

 

Okay -- so if you had a lens that produces a mild foreign-body sensation
(tolerable but annoying -- and it sometimes goes away), and close-up vision
comes and goes, and there are sometimes ghosts (of lights and other bright
objects) -- would that say anything about the fit (I've tried swapping
lenses, new lenses, etc, so its no a problem with a specific lens). My left
eye is fine with the exact same lens -- it's the right one that is having
the problem (and it didn't beofre when I had different lenses that were
larger diameter. My own doc is out of ideas.

Keith

 

Well that's a pretty telling statement: you had a lens previously that
worked and this new one does not. You answered your own question: it is
either the incorrect power, fit or a design not compatible with your
right eye. It is perfectly possible for the left to fit and the right
not.

--LB

--
Dr. Larry Bickford, O.D.
Family Practice Eye Health & Vision Care

The Eyecare Connection
http://www.eyecarecontacts.com
larrydoc at eye-care-contacts dot com (remove -)

 

I'm a bit confused to. Looking back at my notes, my prescription for
standard lens (non-disposable - 40% water) nearly 20 years ago (adult
age) was Base Curve 9.2 Diameter 15 Power -4.25

A recent supply of 1 month disposable lens (55% water) was marked Base
Curve 8.6 Diameter 14 Power -5.00

OK, so I'm 20 years older and my sight has got even worse. (So much
for people who think their sight is stable after about 21 years old
and hve laser / lasik surgery!)

But would you expect the curve of my cornea to alter as I get older?
like from 9.2 to 8.6? Or could that change simply reflect the
super-thin construction of the 1 month disposable lens?

I should add that the doctor says my base curve hasn't changed and
remains 8.9.
Perhaps I'm just given what's in stock at any one time.

The 8.6 lenses don't feel too bad. The 9.2 lenses also felt OK twenty
years ago. Yet I had to visit a hospital on holiday back then and the
optician said the 9.2 lens seemed tight. Looking back, I would expect
it to have been less tight than my current 8.6.

I've always had slightly raised micro veins all around the eye. Could
that be a result of lack of oxygen or the wrong base curve

Regards
George

 

Soft lenses base curves do not match the cornea, the base curve is chosen
that fits best, usually quite a bit flatter than the curve of the cornea.
Different brands of lenses fit differently and one brand may need to be
flatter or steeper than another brand to fit the same eye.

Your old lenses also differed in diameter which affects fit: the larger the
diameter, the flatter the base curve must be. If they were non disposable
fit 20 years ago, they were likely thicker than today's lenses, had a
different water content and likely had less oxygen transmission. Those
factors also affect fit and base curve selection. Older lenses were
designed to move more with the blink, newer lenses drape more and do not
move as much

So, short answer: the 9.2/ 15.0 lens was equivalent to the 8.6/ 14.0 lens
and your eye did not change.

Limbal injection can indicate insufficient oxygen to the cornea; this has
many causes including tight lens, low oxygen lens transmission, high oxygen
demand cornea.

Dr Judy

 

Thank you for your very helpful response. I noticed that the diameter
of my standard non-disposable lens (different brands I believe, and
design trends I assume) was changed over the years. This seems odd, as
I would have expected the diameter to reflect the relatively fixed
size of my pupils or something like that. Perhaps not. A little
knowledge can be a dangerous thing, especially if you are thinking or
ordering lens over the Internet and tinkering with the prescription,
like trying a flatter curve for greater comfort, and lower power to
help with the onset of long sight. Then I could just accept the 'one
size fits all' diameter offered by some lens companies, and suddenly I
have a totally different, totally untried, self-written
'prescription'.

Is there anything which can be done to reverse the very long term
presence of red veins on the whites of my eyes?

Regards
George